Diaper Rash - looks similar to a burn and may scale - if it's really bad it will pimple or blister. At that point I'd give the pediatrician a call. Careful it's not a yeast infection as they are often taken for a diaper rash and can get worse if treated as a rash. Both will definitely be uncomfortable for baby!
Treatment? If it's not too bad I use Buttpaste - Arbonne (only online or through a seller) has an herbal one that is supposedly fantastic!
Heat Rash - Bumpy, red, moise to touch and may or may not bug baby.
Treatment - CORNSTARCH! Cheap and safe. Sprinkle it on dry skin after bath and after changings. This cleared it up in less than a week!